Britain's first high-speed rail line was built on "hugely optimistic" forecasts for passenger demand that never materialised, leaving taxpayers exposed to billions in losses, according to the National Audit Office (NAO).

Revenue expected to cover debts guaranteed by the Department for Transport fell far short, with only two-thirds of the predicted international passengers eventually using the Eurostar to London, a report has found.

The £6.2bn bill for the line was also 18% higher than the original target – although the NAO said that figure compared well with some other rail projects, which came in up to 240% over budget in the case of the west coast mainline upgrade.

Although the report found that the money had delivered a high-performing line that had eventually been successfully sold off to limit taxpayers' exposure, the NAO warned that in future "departments should ensure that demand forecasts are subject to rigorous scrutiny and scepticism".

The findings will give further ammunition to critics of HS2, the £33bn high-speed rail network given the green light by the government earlier this year.

After the economic case for HS2 was challenged repeatedly by opponents and its benefit-cost ratios were subsequently revised downwards, the DfT has largely based its arguments for the project on the need to increase capacity on inter-city railways to meet future demand.

The transport secretary, Justine Greening, announced on Tuesday that the government would publish its plans for the second phase of HS2 in the autumn.

The proposals could include options for stations in Manchester, Leeds, South Yorkshire, the East Midlands and at Heathrow airport. The route for the first phase of the project from London to Birmingham, unveiled earlier this year, is scheduled for completion by 2026, with the Y-shaped tracks further north to follow by 2032/33.

Commenting on the NAO report, Margaret Hodge MP, chair of the House of Commons public accounts committee, said: "Yet again we hear that value for money will depend on uncertain benefits which have not been quantified. It's a sad state of affairs when it comes as no surprise that HS1 was based on dodgy assumptions and bad planning."

A spokesman for the DfT said it had "significantly improved" its passenger forecast modelling since the original work for HS1.

He said: "HS1 was delivered on time and on budget and this report points to the significant benefits it has achieved."